It is known that this type of grinder makes use of centrifugal force to project the substances to be ground onto targets at very high speeds, the whole device being placed under vacuum to avoid the braking of the projected particles by air resistance.
A vacuum grinder incorporates a closed, pressure-resistant enclosure which is evacuated and in the upper part of which is placed a distributor wheel driven in rotation at a high speed.
The wheel is equipped on its axis with a central feed chamber equipped in its upper part with an axial orifice arranged at the bottom of a hopper fed with the substance to be ground by means of a metering device, for example a screw feeder, placed at the outlet of a feed chamber acting as an airlock and enabling the substance to be introduced into the evacuated enclosure.
The distributor wheel is furthermore equipped with a plurality of projection channels whose axes are centered in a median plane perpendicular to the axis and which open inwards into the feed chamber and outwards on to the periphery of the wheel. The substance introduced by the metering device into the central feed chamber is thus entrained by centrifugal action into the channels and projected at the outlet of the latter onto an assembly of plates forming targets and placed all around the wheel, along the side wall of the enclosure. The lower part of the latter is in the shape of a hopper and recovers the fine powder produced by the shattering of the particles of substance which are projected in this way onto the targets by the channels in the wheel.
When the distributor wheel rotates at a sufficient speed, a radial and tangential acceleration is thus produced within the channels, enabling the required velocity to be obtained at the outlet. A contact action between the particles and the wheel, which depends on the speed of rotation, and, consequently, a channel wear phenomenon, is thus produced with these channels, particularly at the outlet of the wheel. This abrasion phenomenon depends on the physical properties of the particles, but is always very considerable as soon as the ejection velocity becomes considerable itself, in view of the high value of the effect of contact between the particles and the wheel and of the relative displacement velocity of the particles in the channels.
Until now, the protective means employed, and particularly the surface treatments, have not made it possible to make the distributor wheel sufficiently resistant, and it is therefore necessary to change the distributor wheel fairly frequently, which is obviously costly, since each wheel has to be made and machined with high accuracy because of its very high rotational speed, which subjects it to high stress. Furthermore, to carry out these operations it is necessary to stop the grinder, which is incompatible with industrial operation in the fields in which continuous production is required, such as cement manufacture or ore grinding.